In hydraulic systems, a hydraulic oil is supplied to hydraulic actuators (such as a hydraulic cylinder and a hydraulic motor) and thereby the hydraulic actuators are operated. Hydraulic systems are widely used in the fields of, for example, construction machinery, industrial vehicles, industrial machinery, and ships and vessels. There are proposed hydraulic systems in which the discharge pressure of a hydraulic pump is detected by a pressure detector and the speed of a variable speed motor configured to drive the hydraulic pump is controlled by using the detected discharge pressure so as to prevent the occurrence of a wasteful amount of discharge at a time when the hydraulic pressure is high.
One example of such a hydraulic system as above is an inverter-driven hydraulic unit disclosed in Patent Literature 1. FIG. 8 shows a configuration of the inverter-driven hydraulic unit. In FIG. 8, an inverter-driven hydraulic unit 1 includes a variable displacement piston pump 2, a variable speed motor 3, an inverter device 4, a pressure sensor 5, and a controller 6. The inverter device 4 and the controller 6 are accommodated in a control panel 7. The variable displacement piston pump 2 includes a pressure adjustment mechanism 9. If the discharge pressure of the variable displacement piston pump 2 reaches a cut-off start pressure, which is slightly lower than a pressure that is set by means of a pressure adjustment screw 15 urged by a spring 10, then the discharge pressure and discharge amount are mechanically controlled by the pressure adjustment mechanism 9. It should be noted that the pressure sensor 5 is configured such that when detecting the value of the discharge pressure, the pressure sensor 5 sends a pressure signal 13, which indicates the detected value, to the controller 6.
As shown in FIG. 9, rotational frequency conditions 12, which correspond to respective operation conditions of the controller 6, are set in advance. The rotational frequency conditions 12 shown in FIG. 9 are represented by a function that is defined by a broken line connecting five points that are set in advance in the controller 6. These five points are set corresponding to hydraulic oil flow rate conditions required by the hydraulic actuator side. Specifically, the rotational frequency of the variable speed motor 3 remains constant at Nc when the discharge pressure of the variable displacement piston pump 2 is in the range from Pa to Pb; the rotational frequency decreases in accordance with an increase in the discharge pressure when the discharge pressure is in the range from Pb to Pc; the rotational frequency is Nb when the discharge pressure is Pc; the rotational frequency further decreases in accordance with an increase in the discharge pressure when the discharge pressure is in the range from Pc to Pd (Pd is a cut-off start pressure); the rotational frequency is Na when the discharge pressure is Pd; and the rotational frequency remains constant at Na when the discharge pressure is in the range from Pd (cut-off start pressure) to Pe (full cut-off pressure). These rotational frequency conditions are set in the controller 6 in advance.
As described above, during a period until the discharge pressure reaches the cut-off start pressure, the discharge amount is controlled by an inverter rotational frequency command from the variable speed motor 3, which is generated based on the discharge pressure detected by the pressure sensor 5 and based on the rotational frequency conditions 12. When the discharge pressure is in the range from the cut-off start pressure to the full cut-off pressure, the discharge amount and discharge pressure are mechanically controlled by the pressure adjustment mechanism 9.